1. Field of the Invention
This invention relates to a photovoltaic device such as a solar cell or a sensor, having a plurality of pin structures superposingly formed.
2. Related Background Art
Thin-film photovoltaic devices making use of amorphous semiconductors in photovoltaic layers have advantages such that large-area films can be produced as compared with single-crystal photovoltaic devices, films can be formed in a smaller thickness and the films can be deposited on any substrate materials, and are considered to be promising.
As a means for improving photoelectric conversion efficiency of the photovoltaic devices making use of amorphous semiconductors, it is attempted, e.g., to incorporate germanium to thereby narrow the band gap to make the sensitivity to long-wavelength light higher.
However, semiconductor materials called amorphous silicon germanium have commonly often caused a decrease in photoconductivity with an increase in the content of germanium. In particular, amorphous silicon germanium containing 35 atm % or more of germanium has often caused a remarkable decrease in photoconductivity. Accordingly, for photovoltaic devices making use of largely germanium-containing amorphous silicon germanium in i-type semiconductor layers, it has been difficult to attain a high photoelectric conversion efficiency.
This is considered due to the fact that localized levels in the band gap increase with an increase in the content of germanium to cause an increase in tail-state levels at the conduction band ends and valence electron band ends, resulting in a short diffusion length of electrons and holes.
As another method for improving photoelectric conversion efficiency of the photovoltaic devices, U.S. Pat. No. 2,949,498 discloses use of what is called a stacked cell in which a plurality of photovoltaic devices having a unit device structure are superposed. The stacked cell is a cell in which devices having different band gaps are superposed to absorb every part of spectra of sunlight in a good efficiency so that the photoelectric conversion efficiency can be improved, and is so designed that the band gap in what is called the bottom layer, which is positioned beneath what is called the top layer, positioned on the light-incident side of the superposed devices, is narrower than the band gap in the top layer. This enables good absorption of spectra of sunlight to bring about an improvement in photoelectric conversion efficiency; see K. Miyachi et al., Proc. 11th, E.C. Photovoltaic Solar Energy Conf., Montreux, Switzerland, 88, 1992, or K. Nomoto et. al., "a-Si Alloy Three-Stacked Solar Cells with High Stabilized Efficiency", 7th Photovoltaic Science and Engineering Conf., Nagoya, 275, 1993.
However, amorphous silicon and amorphous silicon germanium have the problem of a lowering of film quality as a result of irradiation with light, i.e., what is called the Stebler-Ronskie effect. This is a phenomenon peculiar to amorphous semiconductors, which is not seen in crystal systems. Hence, under existing circumstances, they have a poor reliability when used for electric-power purpose, which is an obstacle to their practical application.
Meanwhile, in recent years, studies are made on not only stacked cells of an amorphous/amorphous system but also those of an amorphous/crystalline system, and improvements in photoelectric conversion efficiency of photovoltaic devices are reported; see Hamakawa, Y et al. "Device Physics and Optimum Design of a-Si/Poly Si Tandem Solar Cells", Proceedings of 4th International PVSEC, pp. 403-408, February 1989, A. Shah, H. Keppner, et al., "Intrinsic Microcrystalline Silicon (.mu.-c-Si:H)-A Promising New Thin-film Solar Cell Material", IEEE First World Conference on Photovoltaic Energy Conversion, pp. 409-412, December 1994, or Michell, R. L. et al., "The DOE/SERI Polycrystalline Thin-film Subcontract Program", Proceedings of 20th IEEE Photovoltaic Specialists Conference, pp. 1469-1476, September 1988.
However, taking account of the balance of electric currents generated by light in the stacked cells, the cell on the light-incident side (having a broad band gap) must be made to have a larger thickness, causing a problem that photo-deterioration may greatly occur when films have a large thickness.